Changing type without changing bits

Question

I want to take a stack variable and reinterpret cast it into an unsigned integer type of the same size in bytes. For example, I might want to take double<

Answer 1

If you don't want to have undefined behavior due to violating the aliasing restrictions (C++11 3.10/10) then you need to access the object representations as characters:

template 
int_type caster(const T& value) {
    int_type v;
    static_assert(sizeof(value) == sizeof(v), "");
    std::copy_n(reinterpret_cast(&value),
                sizeof(T),
                reinterpret_cast(&v));
    return v;
}

High quality compilers will optimize the copy away. E.g., this program:

int main() {
    return caster(3.14f);
}

effectively optimizes to return 1078523331; on Intel processors.